1. Field of the Invention
The present invention relates to a device and method for testing the tension in stressed cables of concrete structures, particularly the pre-stressing forces in unbonded tendons in post-tensioned concrete structures.
2. Description of the Related Art
Concrete, while capable of carrying a compressive vertical load, does not carry tensile forces well. To help alleviate this problem, during the last twenty years concrete builders have begun to use unbonded tendons in post-tensioned structures. The unbonded tendons consist of steel cables made up of thin wires wrapped around each other and placed in grease-filled sheathing. The unbonded tendons are anchored at their ends and attached to the concrete solely at these end anchors. Over time such stressed cables have been known to deteriorate, corrode and/or break, thereby resulting in unsafe conditions within the structures, the result being the replacement of the existing post-tensioning system or the abandonment of post-tensioning. For the use of cumbersome external reinforcing. This can lead to a major expense which may often either be deferred or avoided if the actual level of pre-stress in the system is known and monitored over time.
Current testing of concrete includes a visual survey of exposed stressed cables and a penetration test to detect whether these stressed cables are still stressed. This latter test consists of attempting to wedge the flat head of a screwdriver between the individual wires that make up a stressed cable. If penetration is achieved, a stressed cable likely has broken wires or is not fully stressed. Penetration cannot be achieved with a fully stressed cable or when ice or corrosion products wedge a broken strand into place. Accordingly, while these tests are usually informative, they are preliminary in nature, they do not give any indication of the exact level of pre-stress within the stressed cable, and they may actually be misleading.
Several attempts have been made to measure the amount of strength in a structural member. For example, U.S. Pat. No. 5,067,353 to Sersen describes a device for determining the strength of a roof member by determining the amount of deflection of the roof member when a force is applied thereto. The device uses a load base mechanism to apply a force to the roof member and a gauge to measure the resulting deflection when that force is applied.
U.S. Pat. No. 4,501,13 to Mehes et al. describes a test machine for determining the strength of concrete comprising a frame, a breaking cup which is embedded in the concrete before it cures, an extractor head attached to the breaking cup for extraction of the breaking cup once the concrete has solidified, and a force measuring means which applies a tensile force to extract a sample from the cured concrete structure. The device is concerned with testing the strength of the concrete in the concrete structure, and not with measuring the strength of the reinforcement contained therein.
U.S. Pat. No. 3,832,899 to Nicolau describes a dynamometrical deflection measuring method and apparatus. The system uses a compression dynamometer and two articulated bars for measuring the tension in a cable or a chain. The device does not directly measure the deflection in the cable. It is used as a sensor as opposed to a loading device. The device operates by measuring the compression in the articulated bars and assumes that because of geometry, the tension in the cable will be equal. The device assumes that the journalled bars will always remain parallel to the cable in tension, which is not always the case. Furthermore, discrepancy in the geometry will reduce the accuracy of the device. The system is useful in a drillometer on the dead end of an operating cable, or as a torsiometer mounted on the chain of an intermediate transmission.
A review of the prior art therefore suggests that there remains a need for an accurate and inexpensive means to assess the amount of pr e-stress in a stressed cable and particularly a stressed cable of a concrete structure.